Definitely not a big city by American standards (you can walk it in a day and see most things worth seeing, though taking advantage of the great public transport (subway / trams / busses) is worth it, too). Very modern, as most of it was rebuilt after being bombed into oblivion during WWII. That means that there's not all that much of interest to see in the city center (unless you like shopping).The main places to see would probably be the harbour, and the 'modern' (now 20+ years old) architecture of the "[cube houses". Those last are right next to the old harbour, which is probably my favorite picturesque bit of Rotterdam; make sure you don't miss it.

The cube houses are great, I loved visiting them when I was there. You can also head to the Euromast and the Erasmus Bridge, pretty cool views. And if you like football, catch a Feyenoord game or something.

If you are staying for a few more days, you could take the train and visit other cities like Amsterdam and Utrecht.

I could add one suggestion to the ones already made in earlier posts.Buy a ticket for the Spido tour boat around the Rotterdam harbor. It will give you a good impression of both the sheer size of the harbor as well as the variety of activities in and around the harbor area.Have a nice stay in the Netherlands in general, and in Rotterdam in particular.

PSDon't miss Kinderdijk (app. 20 km away from R'dam) once you are here. An incredible collection of cute windmills, in case you're interested in these folcloristic objects too (?).

Rotterdam is all about modernism. So if art & (modern) architecture interest you, then Rotterdam is a fabulous destination.

Apart from the Museum Boijmans-van Beuningen that was mentioned, the nearby Kunsthal (Art Hall) is well worth visiting. It has temporary exhibits, so it does somewhat depend on what they have on, but usually it is good and the building is at least worth having a look at. Just over the road is the NAi (Netherlands Architecture Institute), housed in a beautiful building itself. It has some interesting exhibitions, focussing on the history of Rotterdam. Part of the institute is also the nearby Huis Sonneveld, which you can visit. It's a well-known example of the Nieuwe Bouwen (New Building) period of architecture (early 20th century modern architecture basically).

If you are a fan of design (product, jewellery, etc..), then there are also a lot of interesting shops and design centres in Rotterdam as well. Sorry, it's been a few years since I've been now. Maybe one of the locals can fill you in on that.

Of course, from Rotterdam, you can also visit numerous other towns and sights in the Netherlands. The country is so small, you can get almost anywhere within a couple of hours But that depends on how long you are in the country for.